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William Stallings Data and Computer Communications 7th Edition

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William Stallings Data and Computer Communications 7th Edition Chapter 9 Spread Spectrum Spread Spectrum Analog or digital data Analog signal Spread data over wide ... – PowerPoint PPT presentation

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Title: William Stallings Data and Computer Communications 7th Edition


1
William StallingsData and Computer
Communications7th Edition
  • Chapter 9
  • Spread Spectrum

2
Spread Spectrum
  • Analog or digital data
  • Analog signal
  • Spread data over wide bandwidth
  • Makes jamming and interception harder
  • Frequency hoping
  • Signal broadcast over seemingly random series of
    frequencies
  • Direct Sequence
  • Each bit is represented by multiple bits in
    transmitted signal
  • Chipping code

3
Spread Spectrum Concept
  • Input fed into channel encoder
  • Produces narrow bandwidth analog signal around
    central frequency
  • Signal modulated using sequence of digits
  • Spreading code/sequence
  • Typically generated by pseudonoise/pseudorandom
    number generator
  • Increases bandwidth significantly
  • Spreads spectrum
  • Receiver uses same sequence to demodulate signal
  • Demodulated signal fed into channel decoder

4
General Model of Spread Spectrum System
5
Gains
  • Immunity from various noise and multipath
    distortion
  • Including jamming
  • Can hide/encrypt signals
  • Only receiver who knows spreading code can
    retrieve signal
  • Several users can share same higher bandwidth
    with little interference
  • Cellular telephones
  • Code division multiplexing (CDM)
  • Code division multiple access (CDMA)

6
Pseudorandom Numbers
  • Generated by algorithm using initial seed
  • Deterministic algorithm
  • Not actually random
  • If algorithm good, results pass reasonable tests
    of randomness
  • Need to know algorithm and seed to predict
    sequence

7
Frequency Hopping Spread Spectrum (FHSS)
  • Signal broadcast over seemingly random series of
    frequencies
  • Receiver hops between frequencies in sync with
    transmitter
  • Eavesdroppers hear unintelligible blips
  • Jamming on one frequency affects only a few bits

8
Basic Operation
  • Typically 2k carriers frequencies forming 2k
    channels
  • Channel spacing corresponds with bandwidth of
    input
  • Each channel used for fixed interval
  • 300 ms in IEEE 802.11
  • Some number of bits transmitted using some
    encoding scheme
  • May be fractions of bit (see later)
  • Sequence dictated by spreading code

9
Frequency Hopping Example
10
Frequency Hopping Spread Spectrum System
(Transmitter)
11
Frequency Hopping Spread Spectrum System
(Receiver)
12
Slow and Fast FHSS
  • Frequency shifted every Tc seconds
  • Duration of signal element is Ts seconds
  • Slow FHSS has Tc ? Ts
  • Fast FHSS has Tc lt Ts
  • Generally fast FHSS gives improved performance in
    noise (or jamming)

13
Slow Frequency Hop Spread Spectrum Using MFSK
(M4, k2)
14
Fast Frequency Hop Spread Spectrum Using MFSK
(M4, k2)
15
FHSS Performance Considerations
  • Typically large number of frequencies used
  • Improved resistance to jamming

16
Direct Sequence Spread Spectrum (DSSS)
  • Each bit represented by multiple bits using
    spreading code
  • Spreading code spreads signal across wider
    frequency band
  • In proportion to number of bits used
  • 10 bit spreading code spreads signal across 10
    times bandwidth of 1 bit code
  • One method
  • Combine input with spreading code using XOR
  • Input bit 1 inverts spreading code bit
  • Input zero bit doesnt alter spreading code bit
  • Data rate equal to original spreading code
  • Performance similar to FHSS

17
Direct Sequence Spread Spectrum Example
18
Direct Sequence Spread Spectrum Transmitter
19
Direct Sequence Spread Spectrum Transmitter
20
Direct Sequence Spread Spectrum Using BPSK Example
21
ApproximateSpectrum of DSSS Signal
22
Code Division Multiple Access (CDMA)
  • Multiplexing Technique used with spread spectrum
  • Start with data signal rate D
  • Called bit data rate
  • Break each bit into k chips according to fixed
    pattern specific to each user
  • Users code
  • New channel has chip data rate kD chips per
    second
  • E.g. k6, three users (A,B,C) communicating with
    base receiver R
  • Code for A lt1,-1,-1,1,-1,1gt
  • Code for B lt1,1,-1,-1,1,1gt
  • Code for C lt1,1,-1,1,1,-1gt

23
CDMA Example
24
CDMA Explanation
  • Consider A communicating with base
  • Base knows As code
  • Assume communication already synchronized
  • A wants to send a 1
  • Send chip pattern lt1,-1,-1,1,-1,1gt
  • As code
  • A wants to send 0
  • Send chip pattern lt-1,1,1,-1,1,-1gt
  • Complement of As code
  • Decoder ignores other sources when using As code
    to decode
  • Orthogonal codes

25
CDMA for DSSS
  • n users each using different orthogonal PN
    sequence
  • Modulate each users data stream
  • Using BPSK
  • Multiply by spreading code of user

26
CDMA in a DSSS Environment
27
Seven Channel CDMA Encoding and Decoding
28
Required Reading
  • Stallings chapter 9
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